1. Field of the Invention
The present invention relates to scattering films having wavelength dependency in scattered light distribution and anisotropy in scattered light distributions between the vertical and horizontal directions as well as polarizers and liquid crystal displays using said films.
2. Description of Related Art
Previously, CRTs (Cathode Ray Tubes) have been mainly used for displays in office automation equipments such as word processors, notebook computers and monitors for personal computers, mobile terminals and televisions. Recently, liquid crystal displays have widely replaced CRTs because they are thin and light and consume little power. Liquid crystal displays comprise a liquid crystal cell and a polarizer. The polarizer, which usually consists of a protecting film and a polarizing film, is obtained by dyeing a polarizing film formed of a polyvinyl alcohol film with iodine and stretching it and then laminating a protecting film onto each side of it. For example, transmissive liquid crystal displays may comprise this polarizer on each side of a liquid crystal cell, and optionally one or more optical compensation sheets. On the other hand, reflective liquid crystal displays may comprise a reflector, a liquid crystal cell, one or more optical compensation sheets and a polarizer successively. The liquid crystal cell comprises liquid crystal molecules, two substrates for enclosing them and an electrode layer for applying voltage to the liquid crystal molecules. The liquid crystal cell is switched on and off depending on the alignment of the liquid crystal molecules and can be applied to any of transmissive, reflective and half-transmissive LCDs in various operating modes such as TN (Twisted Nematic), IPS (In-Plane Switching), OCB (Optically Compensatory Bend), VA (Vertically Aligned), ECB (Electrically Controlled Birefringence) and STN (Super Twisted Nematic). However, the color and contrast that can be displayed by conventional liquid crystal displays vary with the angle at which the LCDs are viewed. Thus, viewing angle characteristics of liquid crystal displays have not surpassed those of CRTs.
STN liquid crystal displays using liquid crystal molecules having a twist angle of 180–270° could not achieve high black and white contrast because the birefringence of the liquid crystal polymers resulted in coloration such as dark blue pixels on a yellow-green background. This hue also caused a problem when images were displayed in color by such liquid crystal displays through color filters. An approach to this problem was to improve hue by optical compensation and succeeded in color compensation using a retardation film (e.g., see Nikkei Microdevice, October 1987, page 84), but the color compensation was insufficient partially because the liquid crystal layer and the retardation film have different wavelength distributions at wavelengths other than a specific wavelength to be compensated completely.
Displays in a mode using liquid crystal molecules aligned at a twist angle of 90° (TN mode) (TN-LCDs) show high display contrast with a response time of several tens of milliseconds. This is why many commercially available liquid crystal displays are TN-LCDs. It is known that the optical compensation by retardation films also helps to improve the viewing angle of the TN-LCDs. The retardation films include optical compensation sheets formed of a biaxial film; optical compensation sheets having an optically anisotropic layer containing a discotic compound on a transparent substrate; and optical compensation sheets based on a rod-like liquid crystal compound. Especially, optical compensation sheets based on a disc-shaped compound greatly improved the contrast-viewing angle characteristics of TN-LCDs so that they are widely used in commercially available TN-LCDs, but color shift with viewing angle have not been sufficiently improved.
More recently, wide viewing angle LCD modes have been proposed such as IPS mode using lateral electric field, VA mode in which liquid crystals with negative dielectric anisotropy are vertically aligned, and OCB mode in which liquid crystals are bend-aligned for switching in a birefringence mode with high-speed response. These have a very wide viewing angle and high contrast, and especially S-IPS (Super-IPS) mode further shows a very small color shift with viewing angle by optimizing the electrode shape of IPS mode to improve color shift. However, color shift with viewing angle remains still significant as compared with CRTs.
Color compensation can be certainly achieved by the above retardation films in LCDs switched by controlling polarization. However, contrast is also an important viewing angle characteristic of LCDs, and it is not easy to satisfy both color and contrast performances by using retardation films. Improvements of color in only one direction such as front can be provided by color filters instead of retardation films, but color compensation is separately required at each viewing angle in LCDs because the transmission spectrum of the liquid crystal cell varies with viewing angle.
Methods for controlling the optical path directly from the backlight rather than controlling the optical transmittance in the liquid crystal cell by a retardation film or color filter were also proposed by using a lenticular lens screen for projection liquid crystal displays or an anisotropic light scattering film (see JPA2001-159704) or a prism sheet or the like. These films can be used to distribute light in desired directions, whereby the viewing angle characteristics can be controlled independently from the light control by the liquid crystal cell. However, these films are aimed to control the path of white light but not to control spectral scattering characteristics at each viewing angle, and therefore, their color compensation effect is not complete.
As described above, color-viewing angle characteristics of LCDs could not be improved without impairing other characteristics by conventional color compensation techniques using retardation films or color filters. Even if wide viewing angle LCD modes were used, the color-viewing angle characteristics were inferior to those of CRTs.